Synthesis and Characterization of Single-Phase Metal Dodecaboride Solid Solutions: Zr_{1- x}Y _xB₁₂ and Zr_{1- x}U _xB₁₂

Single-phase metal dodecaboride solid solutions, Zr_0.5Y_0.5B₁₂ and Zr_0.5U_0.5B₁₂, were prepared by arc melting from pure elements. The phase purity and composition were established by powder X-ray diffraction (PXRD), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and ¹⁰B and ¹¹B solid-state nuclear magnetic resonance (NMR) spectroscopy. The effects of carbon addition to Zr_{1- x}Y _xB₁₂ were studied and it was found that carbon causes fast cooling and as a result rapid nucleation of grains, as well as "templating" and patterning effects of the surface morphology. The hardness of the Zr_0.5Y_0.5B₁₂ phase is 47.6 ± 1.7 GPa at 0.49 N load, which is ∼17% higher than that of its parent compounds, ZrB₁₂ and YB₁₂, with hardness values of 41.6 ± 2.6 and 37.5 ± 4.3 GPa, respectively. The hardness of Zr_0.5U_0.5B₁₂ is ∼54% higher than that of its UB₁₂ parent. The dodecaborides were confirmed to be metallic by band structure calculations, diffuse reflectance UV-vis, and solid-state NMR spectroscopies. The nature of the dodecaboride colors-violet for ZrB₁₂ and blue for YB₁₂-can be attributed to charge-transfer. XPS indicates that the metals are in the following oxidation states: Y³⁺, Zr⁴⁺, and U^5+/6+. The superconducting transition temperatures ( T_c) of the dodecaborides were determined to be 4.5 and 6.0 K for YB₁₂ and ZrB₁₂, respectively, as shown by resistivity and superconducting quantum interference device (SQUID) measurements. The T_c of the Zr_0.5Y_0.5B₁₂ solid solution was suppressed to 2.5 K.